With a little help from the internet I figured out how to automatically send a Bcc (blindcopy) of each outgoing e-mail from Apple’s Mail.app (the standard OS X Mail Program) to an e-mail address of my choice (there is a standard setting for sending a copy to the sender address, which is not what I wanted — I like having a copy of everything in my gmail account). As usually, you can do anything you like using the Terminal to change the default properties of the program.

Make sure Mail is closed, and open Terminal (the command line progam which brings you into the machine room of OS X).

In Terminal, type (better, copy and paste) defaults read com.apple.mail UserHeaders
The normal result should be a warning that UserHeaders does not yet exist. If it does exist already, things are going to be a bit more complicated.

Now enter defaults write com.apple.mail UserHeaders '{"Bcc" = "bcc@address"; }' with all the quotation marks as they are, of course using the e-mail-address you want. Done!

The only drawback of this method is that it overrides any Bcc you might enter manually in the future, so if you need to use Bcc for other purposes, this is not for you. By the way, disabling the Bcc is as easy as entering defaults delete com.apple.mail UserHeaders in Terminal (again, provided it didn’t exist before you started).